(a) Field of the Invention
The present invention relates to an automatic transmission, and more particularly, to a powertrain of an automatic transmission.
(b) Description of the Related Art
A multi-stage gearshift mechanism of an automatic transmission includes a plurality of planetary gear sets. A powertrain having such a plurality of planetary gear sets varies the torque in multi-stages and outputs it to an output shaft when receiving a converted engine torque from a torque converter.
The more speeds the powertrain of an automatic transmission has, the better the power performance and fuel consumption. Therefore, it is desirable for powertrains to have as many speeds as possible.
Even for the same number of speeds, durability, power transmission efficiency, and size/weight of a transmission are substantially dependent on how planetary gear sets are arranged. Therefore, research for more structural strength, less power loss, and more compact packaging are continuously being conducted.
Usually, development of a powertrain using planetary gear sets does not devise a wholly new type of planetary gear set. To the contrary, it invokes how single/double pinion planetary gear sets are combined, and how clutches, brakes, and one-way clutches are disposed to the combination of planetary gear sets such that required shift speeds and speed ratios are realized with minimal power loss.
For a manual transmission, too many speeds cause a driver the inconvenience of excessive manual shifting. However, for an automatic transmission, a transmission control unit automatically executes shifting by controlling the operation of the power train, and therefore, more speeds usually implies more merits.
Accordingly, research of four-speed and five-speed powertrains has been undertaken, and recently, a powertrain of an automatic transmission enabling six forward speeds and one reverse speed has been developed. However, in some current transmissions, a first clutch variably connects a third ring gear with a second ring gear acting as the output element. Therefore, because the first clutch, connecting the output element, may be loaded with too much torque, a problem occurs in that durability is reduced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.